While Upgrade Advisor remains available through the Store, Microsoft seems to have shut down the service, breaking the one official upgrade path that existed. Naturally, you can still use the Windows Insider app to upgrade through the Insider Program. Once you do, you can set your phone to the Production ring.

We tested out Upgrade Advisor on a Lumia Icon, 1520, and 640, and none of them worked, despite the fact that the 640 is still an officially supported device. And Microsoft hasn't updated any of the factory images that are available through the Windows Device Recovery Tool, so if you use the app to reset your phone, you'll be back on Windows Phone 8.1 without a proper upgrade path.

If you have a Windows Phone 8.1 handset and you're still considering installing Windows 10 Mobile, now would be the time to do it. While using Windows Insider might be OK as a workaround for now, that won't last forever.